1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to apparatus and method for controlling the flow of water or other confinement media over the laser shocked area of the workpiece during a laser shock peening process.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses one or more radiation pulses, from high power pulsed lasers, to produce an intense shock wave at the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477 entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957 entitled "Material Properties". Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by the instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No.: 5,756,965 entitled "On The Fly Laser Shock Peening"; U.S. Pat. No. 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges"; U.S. Pat. No. 5,569,018 entitled "Technique to prevent or divert cracks"; U.S. Pat. No. 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges"; U.S. Pat. No. 5,492,447 entitled "Laser shock peened rotor components for turbomachinery"; U.S. Pat. No. 5,674,329 entitled "Adhesive tape covered laser shock peening", all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled "Laser Peening System and Method". These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D, to create the beneficial compressive residual stresses. This confining medium also serves as a carrier to remove process generated debris and any unused laser beam energy. Water is an ideal confining medium since it is transparent to the ND:YAG beam wavelength and is easy to implement in production. The water curtain should be kept in continuous contact with the surface of the workpiece or part being LSP'D and at a minimum predetermined thickness or in a range of thicknesses. The water curtain often must be kept at a depth greater than 20 mils.
This water curtain serves the purpose of confining the plasma formed at the top surface of the ablative medium so that the shock wave is driven inward into the metal and not dissipated outward into the air. Within a range of water thickness from about 0 to 80 mil, the effectiveness of the shock wave in the metallic workpiece increases as the water layer thickness increases. Thus, it is important to monitor and control the water thickness at all times during processing so that adequate coverage and thickness is achieved. This can be difficult to do since the workpiece being laser shock peened must be isolated from the operators for safety purposes, and the workpieces typically have complex shapes which preclude many thickness gages. Furthermore, the water flow is dynamic with potentially varying flow rate or pressure.